Touch-sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touching object such as a finger or stylus, either in direct contact, or through proximity (i.e. without contact), with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones. A touch panel that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art.
There are numerous known techniques for providing touch sensitivity, e.g. by incorporating resistive wire grids, capacitive sensors, strain gauges, etc into a touch panel. There are also various types of optical touch systems, which e.g. detect shadows cast by touching objects onto a touch surface, or detect light scattered off the point(s) of touching objects on a touch panel.
One specific type of optical touch system uses projection measurements of light that propagates on a plurality of propagation paths inside a light transmissive panel. The projection measurements thus quantify a property, e.g. power, of the light on the individual propagation paths, when the light has passed the panel. For touch detection, the projection measurements may be processed by simple triangulation, or by more advanced image reconstruction techniques that generate a two-dimensional distribution of disturbances on the touch surface, i.e. an “image” of everything on the touch surface that affects the measured property. The light propagates by total internal reflection (TIR) inside the panel such that a touching object causes the propagating light on one or more propagation paths to be attenuated by so-called frustrated total internal reflection (FTIR). Hence, this type of system is an FTIR-based projection-type touch system (abbreviated “FTIR system” in the following). Examples of such FTIR systems are found in U.S. Pat. No. 3,673,327, U.S. Pat. No. 4,254,333, U.S. Pat. No. 6,972,753, 4, US2006/0114237, US2007/0075648, WO2009/048365, US2009/0153519, WO2010/006882, WO2010/064983, WO2010/134865, WO2012/105893, WO2013/014534, WO2013/191638, WO2014/016685, and WO2014/017973.
FTIR systems offer a number of technical advantages, which may be enhanced by proper implementation. FTIR systems are scalable to large sizes at a relatively modest cost, since the number of optoelectronic components (light emitters and light detectors) scale at most linearly with panel size. Furthermore, FTIR systems require no special sensing elements to be dispersed within or beneath the panel and thus provides a clear and unobstructed view of an underlying display. This may improve contrast and brightness of displayed content and/or enable a reduced power consumption for the display. Still further, FTIR systems readily handle multi-touch events, which means that multiple touch events occur simultaneously. FTIR systems also enable a high resolution at a relatively low cost, since the resolution is determined by the density of propagation paths and the downstream signal processing for recreating the image of the disturbances on the touch surface. Another fundamental advantage of FTIR systems is that only objects in contact with the light transmissive panel affect the propagating light. Thus, the touch sensing is essentially unaffected by objects in the surroundings of the panel, e.g. casting shadows on the panel.
An alternative optical touch-sensing technique based on light scattering is known from U.S. Pat. No. 8,013,845 and U.S. Pat. No. 8,094,136. This technique is based on a light transmissive panel implemented as a multilayered waveguide. One or more light emitters are arranged to illuminate objects that are located in contact with or above the top surface of the waveguide. Thereby, the objects scatter the illuminating light. The waveguide has a signal layer which is spaced from the top surface and optically connected to light detectors. Dedicated microstructures are dispersed across a further layer within the waveguide to guide part of the scattered light into the signal layer such that it is trapped within the signal layer by total internal reflection (TIR) and propagates to the light detectors. The light detectors are configured to indicate the direction and the intensity of the received light. The location of the object in a plane parallel with the top surface may be calculated by triangulation and the distance to the object from the top surface may be calculated based on the light intensity.
While the light scattering technique shares some of the advantages of FTIR systems, it is less capable of handling multi-touch events and the provision of dispersed microstructures may impair the visibility of an underlying display. However, the light scattering technique has the advantage over FTIR systems of being able to not only determine the 2D position of touching objects on the top surface, but also the 3D position of non-touching objects above the top surface. However, in practice it may be difficult to disambiguate between touching and non-touching objects at high precision using the light scattering technique.
Irrespective of the technique used for providing touch sensitivity, most touch systems are in practice limited to detecting objects in contact with the touch surface. They typically have a poor ability of detecting objects that are significantly spaced from the touch surface and/or to determine the distance of non-touching objects to the touch surface with sufficiently high precision.